How can I set up a shock in my Impulse Response Functions to be negative rather than positive?

[NIck Stavr]

I am using VECM and SVAR models and if I am correct always in the IRFs the shock is a 1 st. deviation increase, however, I would like instead of a positive shock to estimate a negative (supply) shock. How can I achieve this? Can anyone here help me on this.
Thank you for your time.